(1) Field of the Invention
This invention relates generally to liquid crystal displays (LCD) and relates more particularly to methods and circuits for storing gamma curve correction data and phase table data in the same memory elements of color super twist nematic (CSTN) display drivers.
(2) Description of the Prior Art
There are many types of liquid crystal displays, each with unique properties. The most common LCD that is used for everyday items like watches and calculators is called the twisted nematic (TN) display. This device consists of a nematic liquid crystal sandwiched between two plates of glass. A special surface treatment is given to the glass such that the molecules are homeotropic yet the director at the top of the sample is perpendicular to the director at the bottom. This configuration sets up a 90-degree twist into the bulk of the liquid crystal, hence the name of the display.
The difference between the ON and OFF voltages in displays with many rows and columns can be very small. For this reason, the TN device is impractical for large information displays with conventional addressing schemes. This problem was solved with the invention of the super-twisted nematic (STN) display. In this device, the director rotates through an angle of 270 degrees, compared with the 90 degrees for the TN cell.
LCD Color super twisted nematic (CSTN) is a display technology based on a passive matrix. It makes useful alternatives to active displays, at less cost. Unlike TFT, CSTN is based on a passive matrix, which is less expensive to produce. New CSTN displays offer 100 ms response times, a 140-degree viewing angle, and high-quality color rivaling TFT displays.
In order to achieve color, it is first necessary to have a display, which is black in one state and white in the other. In a white display, all wavelengths pass through and therefore, all wavelengths can be manipulated to create the desired color. To get full color, each individual pixel is divided into three sub-pixels: red, green and blue (RGB). This means that for each full color pixel, three distinct sub-pixels are employed. These sub-pixels are created by applying color filters, which only allow certain wavelengths to pass through them while absorbing the other wavelengths. Using a combination of red, blue and green sub-pixels of various gray levels, a pixel can be made to appear any number of different colors. By displaying different gray levels of RGB sub-pixels individually, different colors can be achieved. For example, if each R, G, B sub-pixel has 8 gray levels, the maximum number of display colors will be 83 (512 colors).
There are two different methods to address row or COM lines of an LCD. Single-Line Addressing (SLA) or linear scan selects one COM line of the LCD after the other and Multi-Line Addressing (MLA) selects more than one COM lines at the same time. Advantages of MLA are a lower LCD driving voltage requirement which results in power saving, an improved display quality because of faster frame response times and reduced display crosstalk, due also to the lower driving voltages necessary.
The response characteristic between a numeric value expressing a color and the depth of a color input or output actually is expressed by a numeric value referred to as “gamma”. FIG. 1 prior art illustrates the non-linearity of gray levels in LCDs. It shows the transmittance as function of voltage applied.
Any input/output device such as an image scanner, a display device or a printer has its own specific gamma value or gamma curve. Adjusting the gamma value or gamma curve to the specific properties of these devices performs color correction on these devices and is called gamma correction. The gamma value or gamma curve is a parameter indicating the degree of nonlinearity in the intensity of an output signal with respect to an input signal. In any display device, it will be ideal if the output intensity (the brightness of the output in the display device) changes linearly with respect to the change in the value of the input signal. However, the ideal cannot be achieved in a real device.
Usually, liquid crystal devices employ a method in which a storage device serving as a frame memory is provided in a display driver for driving a liquid crystal display panel and display data are read from the storage device and displayed. For example, at present, passive matrix liquid crystal display panels employ such gray scale display methods as the frame rate control (FRC) gray scale method, the voltage gray scale method, and the pulse width modulation (PWM) method. PWM is the subdivision of a COM period into smaller divisions to affect a linear gray scale. In the pulse width modulation method, one horizontal scanning period (1H) selected by a common driver for driving common electrodes (scanning electrodes) is divided into periods of a number that is equal to a prescribed number of gray scales and the period in which an on-waveform is applied is varied in accordance with the gray scale. The pulse width modulation method can control liquid crystal application voltages in such a manner that one horizontal scanning period (1H) is divided into periods of the number of bits constituting each unit of display data for gray scale display with weights given to the respective bits. On the other hand, there may occur a case that in applying voltages to the liquid crystal it is necessary to read out information of only a particular order bit such as MSB information or LSB information. At present, this type of driving method is used in the multi-line addressing (MLA) driving method, for example, in which a plurality of COM electrodes is selected simultaneously.
Frame rate control (FRC) is the sequence of different PWM's in each COM period to affect a linear grey scale. FRC is achieved by tuning RGB sub-pixels on and off over several frame periods. With sufficient frame refreshing time, our human eyes will average the darkness of a pixel so that the individual pixel will show the gray levels required for the color to be displayed. The fixed gray levels are formed by a combination of PWM and FRC. For example: A system that has 128 PWM and 2 FRC has a total possibility of 256 gray levels; 128 gray levels in each of two COM periods.
Phase tables can be used to indicate phases in the sequence of gradation levels of the PWM method to obtain a predetermined gradation level. With use of the table, averaged brightness in each phase table from the first frame to the fourth frame is uniform, and a flicker is difficult to see. The phase table itself is often used in the FRC method.
FIG. 2 prior art shows a block diagram illustrating how the user's input gray data are adapted to output gray levels in order to adapt the LCD driver to the display characteristics. The user's gray data are stored in a RAM 20, e.g. 64 grey levels correspondent to 6 bits gray data input (PWM values). The phase table data are stored hard coded in a ROM 21. Therefore the assignment of gray scale PWM between the individual RFC periods is fixed.
It is a challenge for the designers of passive color LCD systems to optimize the LCD driver to the display characteristics in order to eliminate unwanted display artifacts. There are known patents in the area of passive color LCD:
U.S. Pat. No. (6,836,232 to Bu) proposes a gamma correction apparatus for a liquid crystal display comprising a reference voltage generating circuit and a gamma correction circuit. The reference voltage generating circuit outputs a plurality of reference voltages according to the pixel data. The gamma correction circuit gamma-corrects the pixel data according to the reference voltages. The feature of the invention resides in that the reference voltage generating circuit outputs the corresponding reference voltages to gamma-correct the pixel data according to the positions of the pixels corresponding to the pixel data in the LCD monitor and the display colors of the pixels.
U.S. Pat. No. (6,043,797 to Clifton et al.) discloses a liquid crystal display (LCD) projection unit employing a luminance and color balance system having a lookup table storing multiple sets of gain and/or gamma corrected responses for color balance and luminance control. The lookup table values are determined by measuring an S-curve response of an LCD array for each of a set of R, G, and B input data values, converting the S-curve responses to a corresponding set of gamma responses, and scaling the gamma responses to generate red, green, and blue families of gain and gamma corrected values. Color balance is adjusted by selecting the particular R, G, and B families of gain and gamma corrected values that cause the LCD projection unit to match a predetermined ratio of maximum R, G, and B luminance values. Luminance is adjusted by selecting families of lookup table values that adjust the transmittance of the LCD while maintaining the color balance. The LCD projection unit achieves a uniform luminance and color balance that renders it suitable for use in a multiscreen display system.
U.S. Patent Application Publication (2005/0280624 to Liu) discloses a set of calibration gamma curves, and applying different driving voltages to corresponding positions of an LCD according to the set of calibration gamma curves so that at a same gray scale and at a same fundamental color, brightness is identical and no chromatic aberration occurs in all the positions of the LCD.